This invention relates to a heat exchange apparatus. More specifically, it relates to a heat exchange apparatus comprising a plurality of fluid-tight, hollow filaments made from filled plastic compositions, particularly graphite filled polyfluorinated plastics. More specifically still, it relates to hollow fluid-tight filaments for use in a heat exchange apparatus and a method for producing such filaments in a manner such that they have high thermal conductivity and appreciable mechanical strength.
Plastic heat exchangers, particularly those made from polyfluorinated plastics, such as tetrafluoroethylene or copolymers of tetrafluoroethylene and hexafluoropropylene, have become a useful addition to the array of heat exchanges developed over the past few years, especially in those circumstances where one of the heat exchange media is a corrosive fluid. Plastics, in particular polyfluorinated plastics, however, have a thermal conductivity which is so low that heat exchange devices made from them are not highly efficient. This defect can and has been overcome in part by using a large number of small diameter hollow plastic filaments which are incorporated into a heat exchange bundle having a total surface area large enough so that the heat transfer between the fluid flowing through the interior of the individual filaments and the fluid flowing around the tube bundle is large enough to provide sufficient cooling.
The efficiency of such heat exchange units is still relatively low, however, so there is a need for an improved material for use in such structures which has both the imperviousness to corrosive fluids of the polyfluorinated plastics and increased conductivity. Structures made from polyfluorinated plastics filled with graphite would be expected to have such characteristics, and indeed such structures have been proposed and built. Generally they consist of a very high percentage of graphite, between 70 and 90 percent, with a small amount of a polyfluorinated plastic used as a binder to hold the graphite together. Such structures are mechanically rigid blocks, similar to a block of pure carbon, with holes drilled through them through which a first heat exchange fluid can pass. They are brittle and the surface area exposed to the second heat exchange fluid is generally small, so that even with the improved conductivity of the material, the efficiency of the structure is relatively low.
There is, therefore, a need for an improved combination of polyfluorinated plastics and graphite fillers which can be made into more flexible structures, such as filaments having a high surface area, which in turn can be incorporated into heat exchange structures, such as the one illustrated in U.S. Pat. No. 3,228,456 for a Method and Apparatus Employing Hollow Polyfluorinated Plastic Filaments for Heat Exchange, which issued to P. F. Brown et al. on Jan. 11, 1966. Attempts to formulate flexible filaments form such a combination have, however, run into two related problems which have to date frustrated all attempts to produce useful products. It has been a generally accepted proposition that before the thermal conductivity of the filled plastics can be increased significantly, large amounts of filler would have to be incorporated into them. Once large amounts of filler are used, however, the flexibility and the mechanical durability of the filaments decreases to a point where they are no better, mechanically, than the block structures discussed above.
It is, therefore, an object of the present invention to provide a hollow filament made from a filled plastic composition having a thermal conductivity substantially greater than that of the plastic, and yet having a durability large enough so that such a structure can be used in a conventional heat exchange device. It is a further object of the present invention to provide a method for incorporating thermally conductive fillers into plastic compositions and to produce hollow filaments from such mixtures which have appreciably increased thermal conductivity and appreciable mechanical strength.